The invention relates to a gas scrubbing process in which materials of a first type are selectively scrubbed out of a gas mixture conveyed in countercurrent to a physically acting scrubbing medium in a first scrubbing step and materials of a second type are selectively scrubbed out in a subsequent second scrubbing step, where a part of the scrubbing medium which has been loaded with materials of the second type in the second scrubbing step and is free of materials of the first type is used in the first scrubbing step so as to form a scrubbing medium stream which is loaded with materials of the first type and of the second type, in the regeneration of which a partially regenerated scrubbing medium stream (semilean) which contains materials of the first type and of the second type and has a lower content of materials of the first type than the scrubbing medium stream which has been loaded in the first scrubbing step is produced.
In addition, the invention relates to an apparatus for carrying out the process of the invention.
Physical gas scrubs utilize the ability of liquids to absorb gaseous materials and keep them in solution without chemically binding the gases. How well a gas is absorbed by a liquid is expressed by the solubility coefficient: the better the gas dissolves in the liquid, the greater is its solubility coefficient. The solubility coefficient is temperature-dependent and generally increases with decreasing temperature.
If a gas component i is to be dissolved out of a gas mixture, by physical scrubbing, it is for this purpose necessary to use a minimum amount Wmin of the liquid employed as scrubbing medium, which can be calculated very readily by means of the following formula:Wmin=W/(p·λi)
In the formula, V is the total amount of the gas mixture, p is the pressure prevailing in the gas mixture and λ is the solubility coefficient of the gas component to be scrubbed out in respect of the scrubbing medium used.
With the proviso that the solubility coefficients of the components of a gas mixture differ sufficiently greatly, it is possible to separate off, i.e, selectively remove, the gas component having the greatest solubility coefficient largely independently of the other gas components in a scrubbing step by appropriate adaption of the amount of scrubbing medium. Following the same principle, further gas components or groups of gas components having similar solubility coefficients can be selectively scrubbed out using larger amounts of scrubbing medium in subsequent scrubbing steps.
The gas components which have been scrubbed out are removed from the loaded scrubbing medium after the gas scrub, by which means the scrubbing medium is regenerated. The regenerated scrubbing medium is normally reused in the gas scrub, while the gas components which have been scrubbed out are either disposed of or passed to a profitable use.
For the purification of crude synthesis gases which are produced on an industrial scale from coal and/or hydrocarbon feeds in gasification plants, for example by reforming using steam or by partial oxidation, and generally contain some undesirable constituents such as wafer, carbon dioxide (CO2), hydrogen sulfide (H2S) and carbon oxide sulfide (COS), preference is given to using physical gas scrubs. These processes are attractive since the crude synthesis gases are nowadays usually produced under high pressure and the effectiveness of physical gas scrubs to a first approximation increases linearly with the operating pressure. The methanol scrub is of particular importance for the purification of crude synthesis gases. It utilizes the fact that the solubility coefficients of H2S, COS and CO2 in liquid, low-temperature methanol differ by several orders of magnitude from those of hydrogen (H2) and carbon monoxide (CO). The fact that the sulfur components have very similar solubility coefficients which are significantly greater than the solubility coefficient of CO2 in liquid, low-temperature methanol is exploited in order to separate off the two types of material selectively from crude synthesis gas and possibly utilize them profitably.
According to the prior art, the crude synthesis gas is subjected to a gas scrub which consists of two scrubbing steps and in which liquid methanol is used as physically acting scrubbing medium in order to separate off sulfur components and carbon dioxide selectively. In the first scrubbing step, in which the sulfur components are separated off from the crude synthesis gas, part of the methanol which has been loaded with CO2 in the second scrubbing step but is free of sulfur is used as scrubbing medium. The methanol which has been loaded with sulfur components and CO2 in the first scrubbing step is subsequently completely regenerated, with the dissolved materials being separated off with a considerable outlay in order to produce methanol of scrubbing medium purity. The methanol of scrubbing medium purity is subsequently recirculated and reused as scrubbing medium in the second scrubbing step in order to separate CO2 from the crude synthesis gas.